The present invention is directed to an electrochemical micro-sensor device for detecting or monitoring sulfur dioxide. More particularly, the invention is directed to a thick film electrochemical micro-sensor device capable of detecting and monitoring sulfur dioxide emissions.
Acidic deposition, which includes acid rain, acid snow, and even acidic dusts, is currently a major environmental issue. It is not only a critical threat to the natural world but also to the world of man. In recent years, acidic deposition has increased greatly which, in turn, amplifies its effects. It has depleted the fisheries in various lakes around the world. It has also caused the destruction of great forests around the world, especially in Germany. Moreover, acidic deposition is, in itself, damaging human health in various locations. Finally, acidic deposition poses a great threat to the preservation of human history. All these problems will be discussed more thoroughly in the following paragraphs.
First of all, acidic deposition is a main factor behind the depletion of aquatic life in various lakes whether natural or man-made. When acidic deposition occurs, the acidic chemicals mix with the water in the lakes, gradually increasing the acidity of the lakes. Because all species of aquatic life forms can only reproduce and certain eggs can only hatch at certain acidic levels, these species are unable to reproduce regularly if they can reproduce at all. This is a major concern for various fish species as well as certain small shrimps and mollusks. Furthermore, the acidic chemicals from repeated acidic deposition leaches dangerous metals, such as mercury and lead, from the soil at the lake bottoms and into the lake waters. Eventually, the concentration of these toxic-metal substances in the lake reach a certain level where the organisms living within the lake waters start dying off specie by specie. After a certain time, these acidified lakes will look crystal clear, but that will be because there will be nothing living in the lakes except maybe a couple of algae species.
Acidic deposition is also a main cause behind the devastation of major forests around the world, especially in Germany. The acidic deposition descends into the forests and permeates through the soil. Similar to its effects on the lakes, the acidic chemicals once again release dangerous chemicals, such as aluminum, except this time from the soil. These metals instigate a slow process which kills the trees. Another effect of acidic deposition is that the acidic chemicals, once in the soil, displace the nutrients, such as calcium, which are important to trees and other plant life, from the ground. Because of this displacement, the plant life in such forests is unable to absorb the nutrients necessary for its survival from the soil at forest bottoms.
Acidic deposition is not only an environmental problem but also a problem for humans because it is damaging to human health as well. First of all, the acidic chemicals from acidic deposition, especially from acid rain and acid snow, have the ability to leach toxic metals, such as copper and lead, into drinking water for humans. The presence of these metals in everyday drinking water would obviously harm the health of humans who intake this water. Moreover, the acidic chemicals have been found to be the main sources behind the outbreaks of gastroenteritis in places such as the Adirondack Mountains. Furthermore, when in high concentrations, the acids are capable of irritating the human respiratory system causing problems in everyday functions such as breathing. Finally, medical institutions have also suspected that the acidic content of rain, snow, and dust may be the cause of some types of chronic bronchitis and emphysema which would eventually lead to chronic heart disease.
Lastly, another major problem initiated by acid rain is the destruction of human history by the deterioration of man-made historical architectures. History buildings, such as the magnificent Acropolis in Greece and the stunning Pantheon in Rome, have survived the weathering from rain and snow and wind for hundreds of centuries. However, in recent years, with the rapid increase in acidity in rain, snow, and even dust, these famous buildings representing the progress and history of mankind have deteriorated at higher and higher rates. This deterioration is an obvious effect of the corrosive acidic content of the rain. Because these structures are made mostly in limestone and marble which are basic, the acid reacts with the stones and corrodes the surfaces. However, the deterioration not only occurs with stone monuments and buildings, but also affects the metal structures by increasing the rate of oxidation or rust of the metal. Therefore, acid deposition not only plays a significant part in the destruction of the environment but it also affects humans through damaging human health and deteriorating the traces of stepping stones in human history.
There are three main types of acidic deposition. Acidic dust or ashes are acidic chemicals that descend from the sky in the form of dry flaky solids. Another type of acidic deposition is acidic snow or sleet. This type of acidic deposition occurs with the freezing or crystallization of the acid rain while or before it precipitates from the atmosphere. Finally, most importantly, there is acid rain. Acid rain occurs when the acidic compounds released in gaseous form make contact with the moisture in the atmosphere. Then, when the moisture reaches a certain level and descends from the sky, the acidic compounds have already combined with the rain droplets making the rain acidic.
There are two main groups of gases that contribute to the formation of acidic deposition. The first group includes the group of nitrogen oxides. Its main contributors are automobiles. Another, which is the major contributor to acidic deposition, is sulfur dioxide which is released mainly from the smokestacks of industrial buildings. Outside of these two groups, there are some other minor groups of gases that also contribute to acidic deposition, such as carbon dioxide. When these various acidic gases are released into the atmosphere, they come in contact with the moisture in the atmosphere. Then, with the water, the gases form acidic solutions which fall to the ground in the form of acid rain, acid snow, or acidic dust as described in the previous paragraph.
As mentioned previously, the main contributor to acidic deposition is sulfur dioxide. Sulfur dioxide is usually produced as a result of burning coal and oil. It is also often produced as a superfluous product by the processes in refineries, pulp and paper mills, as well as metal smelters. When released into the atmosphere, sulfur dioxide and other oxides of sulfur have the ability to form sulfates with the oxygen as well as aerosols of sulfurous and sulfuric acids with water vapor. All these chemicals eventually come down to the ground in various forms of acidic deposition. Therefore, sulfur dioxide harms the environment as well as people in the form of acidic deposition. However, in high concentrations, the sulfur dioxide, by itself, is also a problem. It, by itself, possesses the ability to aggravate already present respiratory and cardiovascular diseases including asthma, bronchitis, and emphysema.
Therefore, in order to help in reducing acid rain, the amount of sulfur dioxide released needs to be reduced. Although there are currently processes such as the use of scrubbers or calcium carbonate released into the sulfur dioxide emissions to neutralize the gas, the processes are expensive, and they also cannot monitor the sulfur dioxide levels. Therefore, it is believed that the most effective way to reduce sulfur dioxide would be through monitoring the sulfur dioxide released from industrial smokestacks. This could lead to a more effective way to regulate sulfur dioxide emission levels. There are many ways to measure sulfur dioxide, and one of these ways is through using our newly developed micro-sensor and sensor technology.
There are many types of sensor technology used for detecting gaseous sulfur dioxide. These sensors can determine the presence of and, usually, the amount of sulfur dioxide in an environment. Of these sensors, spectrophotometric analyzers, conductometric sensors, solid electrolyte electrochemical cells, piezoelectric crystal detectors, and interdigital capacitors (IDCs) are more commonly used. Each type along with its advantages and disadvantages will be discussed in the following paragraphs.
Spectrophotometric analysis is currently the standard technology used for the detection of sulfur dioxide. Spectrophotometric analyzers use either ultraviolet or infrared light to determine the presence of as well as the amount of sulfur dioxide present. Although spectrophotometric analyzers may be the standard method of detection, they often suffer from the interfering absorptions from other sources. Moreover, on top of the fact that they are very expensive commercially to manufacture, and the operation of the analyzers is elaborate as well as complicated in that these analyzers contain mechanical moving parts.
Another method of sensing sulfur dioxide is through the application of conductometric sensors. In conductometric sensors, the sulfur dioxide gas diffuses through a gas-permeable membrane and reaches equilibration in a layer of water. Within the thin layer of water, conductometric sensors are positioned. When the sulfur dioxide gas reaches equilibration in the thin water layer, these conductometric electrodes determine the conductance of the sulfur dioxide gas. Using the conductivity measured, the sulfur dioxide level can be determined in ninety to one hundred and twenty seconds. The advantages of these conductometric sensors include high sensitivity to sulfur dioxide. Moreover, they are simpler to operate than the spectrophotometric analyzers. However, they also have their disadvantages, which include being non-specific, meaning that it is difficult for this kind of sensor to differentiate sulfur dioxide from other gases. Furthermore, another disadvantage is their constant need for extensive maintenance in comparison to the maintenance needs of other methods.
Another method for detection of sulfur dioxide employs solid electrolyte electrochemical cells. These cells contain a Na2SO4xe2x80x94Li2SO4xe2x80x94Y2(SO4)3xe2x80x94SO3 solid electrolyte and a solid reference electrode. These cells determine sulfur dioxide levels through measuring the electromotive force (EMF). They are more compact and less expensive than spectrophotometric analyzers. They are also capable of detecting sulfur dioxide continuously. Moreover, they have the capability to respond only to sulfur dioxide gas even in the parts per billion (ppb) range. However, these solid electrolyte sensors have disadvantages as well. Their disadvantages include the fact that they need a regulated supply of a reference gas mixture containing both sulfur dioxide and air. Another main disadvantage they possess is that these sensors can only operate at the most optimum quality within the restricted temperature range of 783 K and 833 K.
There are also piezoelectric crystal detectors which make up another sector of sensors used for sulfur dioxide detection. When sulfur dioxide gas is bubbled through a mercurous nitrate solution, a mercury displacement reaction occurs producing mercury vapor. Because of the capability of gold to absorb as well as amalgamate mercury, a gold-coated piezoelectric crystal is used. When the mercury vapor reaches the gold-coated piezoelectric crystal, the crystal detects the mercury vapor because a mercury alloy or amalgam is formed. By measuring and monitoring the mercury using the piezoelectric crystal, the concentration of sulfur dioxide in the sample of air can be determined as well. This method is advantageous in that it has good sensitivity for sulfur dioxide because it can detect sulfur dioxide both in the parts per billion (ppb) and in the parts per million (ppm) ranges. Furthermore, these detectors also have good selectivity for sulfur dioxide in that they are capable of distinguishing sulfur dioxide from the other gases in the ambient air. However, these piezoelectric crystal detectors also suffer from disadvantages. The main disadvantage is the concern that they may produce and emit hazardous mercury gas which can cause serious damage and problems.
Finally, there is a method of detecting sulfur dioxide which employs interdigital capacitors or IDCs. These IDCs have organic absorption centers, which are often constructed using organically modified silicates. These capacitors are claimed to have good selectivity for sulfur dioxide, which is an important advantage. However, there is also one big concern facing this method. Because this method is relatively new, there have been insufficient experimental data collected supporting this claim of good selectivity.
More recent technology of electrochemical sensors, involves two main basic types of micro-sensors, thick film and thin film. Thin film electrochemical sensors apply vapor deposition to produce the sensing elements, which makes the sensors more expensive to produce. However, the thickness of these thin film micro-sensors is only a few microns. The other type of electrochemical micro-sensors is the technology of thick-film sensors which apply a silkscreen-like process for printing the sensors, and because of this, the thickness of these micro-sensors is approximately 0.02 inches, making them thicker than thin film sensors.
Of these thick film electrochemical sensors, there are two main branches of configuration. One employs a two-electrode configuration while the other one applies a three-electrode configuration that is more accurate because it includes a reference electrode. A two-electrode configuration combines the counter and the reference electrode into one electrode while the three-electrode one separates the two electrodes making the results more accurate. For both types of sensors, the Gibbs free energy is first calculated from the oxidation-reduction or redox reaction that occurs when the substance being detected is at the working electrode. Then the calculated Gibbs free energy will be used to determine the necessary potential voltage to apply to the working and counter electrodes in relation to the voltage used in the reference electrode to allow the specific redox equation to occur. This calculated potential voltage is applied to the working and counter electrodes of the micro-sensor against the EMF (electromotive force) of the silver-silver-chloride electrode. Furthermore, the corresponding current produced is measured. After many tests are run with the sensor under various concentrations, the results are used to calibrate the sensors by fitting a linear line of concentrations versus current to the data, which can then be used in order to quantify the sulfur dioxide being measured.
This technology of thick film electrochemical micro-sensors has been used in various fields because of its cost efficient as well as uncomplicated method of manufacture and use. They have been used to detected acidity in waters and even for monitoring human health. For example, they have been used in the project CHIME which uses this type of sensors for monitoring the actions of the heart, among which is the rate of heart beats of babies who range from just born to a couple of years old. However, being a relatively new technology, thick-film electrochemical micro-sensors have not yet been applied to either detecting sulfur dioxide or determining the concentration of sulfur dioxide in gaseous samples.
It is therefore an object of the present invention to provide a thick film electrochemical micro-sensor for detecting sulfur dioxide.
The present invention provides an effective and economical electrochemical micro-sensor device for detecting or monitoring sulfur dioxide comprising a substrate supporting an arrangement of a working electrode, a reference electrode, and a counter electrode, wherein a first portion of the electrodes is covered with an insulator, and a second portion of the electrodes is covered with an electrolyte, and wherein the electrodes are applied to the substrate using a thick film technique.
The present invention more optimally provides an electrochemical micro-sensor device for detecting or monitoring sulfur dioxide comprising a substrate containing an arrangement of a working electrode, a reference electrode, and a counter electrode, wherein a first portion of the electrodes is covered with an insulator, and a second portion of the electrodes is covered with an electrolyte, wherein the electrodes and the insulator are applied to the substrate using a thick film technique, and wherein the sensing portions of the working and counter electrodes are disposed adjacent to each other, with a gap therebetween of less than or equal to about 0.2 inches, in an optimum configuration, as described herein.
The present invention further provides a method of detecting sulfur dioxide in an emission gas comprising contacting the emission gas with the inventive sensor, measuring the current output of the sensor, determining if the current output indicates the presence of sulfur dioxide, and generating a signal. This signal can then be used to activate a display device, a recording means, an alarm device, and/or a compensating means.
The present invention additionally provides a method of detecting sulfur dioxide in an emission gas using at least two micro-sensor devices in a differential mode of operation. The method comprises contacting the emission gas with a first inventive sensor, measuring the current output of the sensor, generating a first signal based on the current output of the sensor, providing at least a second inventive sensor, which has been adapted to detect interference from other chemical species, contacting the emission gas with the second sensor, measuring the current output of the second sensor, generating a second signal, and subtracting the second signal from the first signal. This signal can then be used to activate a display device, a recording means, an alarm device, and/or a compensating means.
It has been found that as the size of the working electrode of the micro-sensors increases, the sensitivity as well as the current output increases because the surface area at which the reaction takes place increases. The efficiency of the micro-sensor device is thus increased as the surface area of the electrodes increases. Novel electrode configurations designed to maximize this effect were tested, and the results are reported herein, along with the preferred electrode configurations.
It has further been found that as the gap between the working and counter electrodes decreases, the sensitivity as well as the output current increases, theoretically because the electrons have less resistance when transferring from the counter electrode to the working electrode. The efficiency of the micro-sensor device is thus increased as the gap between the working and counter electrodes decreases. Novel electrode configurations designed to maximize this effect were tested, and the results are reported herein, along with the preferred electrode configurations.
It has also been found that as the length of the working electrode and counter electrode adjacent to one another increases, the sensitivity as well as the current output increases because there is a greater length of the region where the electrons have less resistance moving from the working electrode to the counter electrode. The efficiency of the micro-sensor device is thus increased as the length of the region where the working electrode and counter electrode are adjacent to one another increases. Novel electrode configurations designed to maximize this effect were tested, and the results are reported herein, along with the preferred electrode configurations.
There exists a linear relationship between the current output and the concentration of the sulfur dioxide because, as the concentration increases, the amount of electrons transferred increases as well, contributing to a higher current output. This linear relationship allows the electrochemical micro-sensor device of the present invention to detect and quantitatively monitor sulfur dioxide emissions.